Historically, helicopter main rotor hubs have been made of high strength, lightweight, critical metals or alloys. Although these metal components have performed adequately, there are a number of drawbacks inherent to these materials. Three important areas where these materials possess less than optimum features are weight, availability and damage tolerance.
Weight has always been a consideration in helicopter construction and with increasing fuel costs it has become a primary objective, i.e. to reduce the weight of the overall helicopter by using lighter materials. At present aluminum and titanium are used extensively because of their light weight and strength, however, there is a constant search for lighter and stronger materials. In addition, these lightweight metals are classified as "critical" materials with their primary availability being through importation. As has been demonstrated by the oil embargoes of past years, reliance on foreign sources for these materials is not desirable. Furthermore, these metals do not impart a damage tolerance to their components. That is, when a metal component starts to weaken, through fatigue or otherwise, cracks are generated. These cracks continue to grow quickly as there is nothing to stop their propagation and the component part can fail completely. As can be appreciated, this lack of damage tolerance can be disastrous in a helicopter.
In order to overcome the shortcomings of such metal components, the industry has taken two approaches. One is o build a redundant component so that should one fail, the other will allow for safe landing. The second is to overdesign the particular part such that it would have much greater strength than would normally be required under normal circumstances. Both of these approaches add weight to the aircraft as well as increased cost and reliance on critical metals.
Recently, composite materials have been used as replacement parts for many metal components due to their light weight and relatively low cost. For example, composite materials are now being used in main structural components such as main rotor blades and tail rotor assemblies on helicopters. However, it is not always practical to replace a metal component with a composite material due to particular design considerations and shortcomings in the composite physical properties.
Composite rotor hubs have been designed using a laminated structure of fiber reinforced resin (e.g., note British Patent No. 2,092,541). Such hubs are of such complicated design as to make them costly to fabricate, and having fewer component parts, produce an increased number of failure points. Commonly assigned U.S. Pat. No. 4,568,245 describes another composite main rotor hub.
However, there is still a constant search in this art for damage tolerant, relatively inexpensive and lightweight composite components capable of withstanding the forces developed in a helicopter rotor hub assembly.